This invention relates to laminates containing a rubber surface and in particular to laminates which are useful as printing blankets for the printing industry. The invention also relates to a method of preparing such laminates which are of the compressible variety.
The use of blankets in offset printing for transferring the ink from a printing plate to the paper is well known. Printing blankets must be carefully designed so that the surface of the blanket is not damaged either by the mechanical contact with the parts of the press or by chemical reaction with the ink or ink ingredients. These repeated contacts cause a certain amount of compression of the blanket which must be within proper limits so that the image is properly transferred without causing permanent deformation of the blanket. An important feature of offset printing blankets, therefore, is the ability of the blanket to return to its original thickness upon repeated use and to provide constant image transfer.
In addition to the problems caused by variations of the printing apparatus, conventional printing blankets may contain variations in thicknesses which are formed during the manufacturing steps. The existence of high or low spots in blankets may create uneven reproductions on the finished product.
The printing blankets which are currently used in the industry generally comprise a base material which provides integrity on the blanket and a working surface or top layer of an elastomeric material made of natural or synthetic rubber. The base material may comprise one or more fabric plies and a layer of a soft highly resilient rubber backing layer. The working surface is a void-free layer of elastomeric material which actually contacts the ink.
The usual method for applying the working layer of elastomeric material to the laminate is by calendering or spreading rubber in successive thin layers until a desired thickness of rubber has been deposited. The assembly then is cured to provide the finished blanket. Such blankets are acceptable but often lack the necessary compressibility or compression set characteristics required in many applications. Moreover, the spreading of successive thin layers of rubber to form the top coat is tedious and often results in surface irregularities which cannot be completely removed by surface treatments such as grinding.
Although blankets have been manufactured with improved compression set characteristics through the use of special plasticizers, textile fibers, foam backing, etc., ideal results have not been obtained because of problems such as uneven compressibility of the surface coating resulting from nonuniform closed cell structure in the backing. Also, backing materials containing cell structures formed by blowing processes have not always contained uniform cellular structures.
The problem with the quality of the printing obtained through the use of the known printing blankets has been magnified by the recent developments in printing technology such as by the design and introduction of very large presses running at ever-increasing speed and new developments in inks and paper.
U.S. Pat. No. 3,795,568 describes a typical procedure which has been used for preparing printing blankets. The normal procedure involves the formation of the laminate by successive formation of the various layers. For example, two plies of fabric are bonded together by a neoprene cement and placed upon a conventional blanket manufacturing machine. A compressible intermediate layer is formed on the second fabric ply by spreading a mixture of a rubber compound, solvent and foam rubber particles onto the fabric and exposing the mixture to an elevated temperature to remove the solvent. A third adhesive coated fabric ply is placed over the spread layer and the assembly is passed through a spreading machine where layers of rubber are applied to the surface by repeated steps until a layer of about 0.01 inch is built up. The assembly then is cured at an elevated temperature.
Printing blankets which have been used in the past have exhibited moderate resistance to the vehicles used in the inks. The blankets have not been easy to clean, swell considerably after extended cleaning, and are not impervious to ozone attack.
Thus, there continues to be a need in the printing industry for a printing blanket which overcomes these problems and deficiencies without a compromise in the desirable and necessary properties of printability, ink release and dimensional stability.